Wood in the Landscape: Decks Part II
Inorganic soils, those with high contents of sand, gravel, or inorganic silts have a bearing capacity of 1500 psf or more (check with the local building department of conservation agent).
In this soil condition, the bearing capacity for a 12-inch by 12-inch footing is 1500 lbs. If the soils contain higher amounts of clay or organic material, the bearing capacity may be less, requiring a greater footing footprint.
Computing Footing Size
The dimensions for concrete footings carrying normal loads in good soils are 12 by 12 inch to 14 by 14 inch and for round footings, a diameter of 14 to 16 inches.
In most situations, a 6-inch deep spread footing is adequate, but for heavy loads or for a spread footing greater than 16 inches on a side, the depth will increase to 8 inches.
The number of footings required is related to the tributary loads that will be carried. For large decks, an engineer should be consulted, but these figures provide a general rule of thumb for footing designs.
The footing, typically a poured concrete structure, must sit below the point where soil freezes in winter. Information on the depth of freezing can be acquired from local building departments. Frost heave is a condition of water, temperature, and soil permeability that results when water in the soil freezes and expands. It can lift the footing, resulting in racking or structural failure of the deck.
In areas prone to frost heave or with poor drainage, additional drainage material such as drain rock can be placed below the footings to reduce water retention in the soil. For additional strength, a grid of rebar can be embedded into the footing 8 inches on center.
In the illustration, B is 76 X 72 inches = 5472 square inches; divided by 144 = 38 square feet. To calculate the pressure, multiply 38 by 50 psf. which equals 1900 pounds. Because the soil-bearing capacity is 1500 psf. the spread must be greater than 12 X12 inches. Dividing 1900 pounds by 1500 psf gives a footing size of 1.26 square feet or 1.12 feet square. Converting to inches and rounding up, footing dimensions of 14 X 14 inches will be adequate.
Building the Piers
The simplest footing is a pier block, a precast battered square concrete block with an embedded post anchor, on which the post is bolted. Unless set below grade, piers will not resist any frost heave, and because of their size, have limits on their load-carrying capacity. Some codes require a more permanent cast-in-place footing (check with local codes).
Concrete piers, required by code in many parts of the country, are not susceptible to insect infestations and are decay-proof and very strong. The pier is typically poured on top of a spread footing which has been stubbed out with reinforcing bars to tie the pier to the footing.
The spread footing is a 6- to 8-inch-thick square form, typically 6 to 8 inches greater in width than the pier diameter. After the spread footing pour has set, a concrete pier form is placed on top of the footing. The form can be constructed of wood to pour a square or rectangular column.
Alternately, cylindrical paper forms, "sono" tubes, can be used, which are available in diameter sizes from 4 inches, increasing in 2-inch increments up to 18 inches and in diameters of 24 and 36 inches.
An 8-inch diameter pier, with two to four L-shaped vertical reinforcing bars embedded in the footing, is usually adequate for a 4 X 4 wood post. A 10- to 12-inch-diameter pier is recommended for a 6 X 6 post. After the concrete is poured, a galvanized steel post anchor can be embedded into the top of the form to receive a post or beam.
Installing a Posts
Though this is not required by most codes, the post anchor separates the end grain of the post from the concrete surface and creates a half-inch air gap below the post that ensures good ventilation below the end grain of the wood post.
The top of the concrete pier should be hand-formed to provide an adequate pitch sloping away from the post base, reducing any water build up that could infiltrate the end grain.
The intermediary wood post transfers the load from the beam and joists to the pier or footing. If the finished elevation of the deck is less than 1 to 2 feet from grade, the concrete piers typically extend above grade to receive the beam, eliminating the wood post.
If a wood post is used, a 4 X 4 post is often adequate to transfer the loads. However many designers specify larger 6 X 6 posts and double-notch the post on either side to provide a 1.5-inch seat that will receive a sandwich beam. In this method a 2.5-inch tongue is left on the post that protrudes up through the beam and through which the beam and post can be bolted together.
The post need not be one solid member, especially if it is greater than 8 inches in either dimension. Timbers of this dimension can be expensive or difficult to find. An alternative is to build up a post using standard 2 X dimensional lumber. A 6 X 6 post can be fabricated from four pieces of 2 X 6, glued and laminated.
Next week we continue the five-part series with Part III: Decks, Beams and Joists.
Daniel Winterbottom teaches in the University of Washington's Department of Landscape Architecture. His interests focus on urban and community landscape design, vernacular landscapes, therapeutic gardens, sustainable design, and the craft and detailing of built forms. He is principal of Winterbottom Design, a Seattle-based landscape architecture and site planning firm.
Wood in the Landscape: A Practical Guide to Specification and Design, copyright 2000 by Daniel Winterbottom, is available at bookstores and at Amazon. To order from the publisher, visit John Wiley & Sons or call 800-225-5945.